/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* merge(ListNode* l1, ListNode* l2)
    {
        if (!l1) return l2;
        if (!l2) return l1;
        if (l1->val >= l2->val)
        {
            l2->next = merge(l2->next, l1);
            return l2;
        }
        else
        {
            l1->next = merge(l1->next, l2);
            return l1;
        }

    }

    ListNode* mergeKLists(vector<ListNode*>& lists)
    {
        if (lists.size() == 0) return nullptr;
        if (lists.size() == 1) return lists[0];
        ListNode* newhead = lists[0];
        for (int i = 1; i < lists.size(); i++)
        {
            newhead = merge(newhead, lists[i]);
        }
        return newhead;
    }
};